How to animate first item loaded using jquery?

How to animate first item loaded using jquery?

I am using the load function for my links to load html files inside a div.
I also use slideDown to animate the content, but it doesn't work for the
first html loaded. How can I fix that?
http://jsfiddle.net/QNQv9/
$(document).ready(function() {
$(".iframe").on("click", function(e) {
e.preventDefault();
$iframe.load($(this).attr('href'));
$iframe.slideDown(500)
});
});